Preparation of chemical pulp from wood chips



United States Patent 01 3,448,004 Patented June 3, 1969 ice US. Cl. 16275 12 Claims ABSTRACT OF THE DISCLOSURE The addition of an N,N-dimethylamide of a straight chain 18 carbon atom carboxylic acid to the cooking chemicals used in the preparation of chemical pulp from wood chips reduces the amount of cooking chemicals required in the process accompanied by increased yield and improved quality of the chemical pulp.

This is a continuation-in-part of application Ser. No. 544,396 filed Apr. 22, 1966 and now abandoned.

This invention relates to the preparation of chemical pulp from cellulosic material by the surfate and sulfite processes. More particularly, the present invention constitutes an improvement over known processes of producing chemical pulp by digesting cellulosic materials such as wood chips by the sulfate and sulfite processes.

Since the methods of producing chemical pulp by the sulfate and sulfite processes are Well known by those skilled in the art, a description thereof will not be repeated here. For a detailed description of these processes, reference is made to Sven A. Rydholm, Pul ing Processes, Interscience Publishers, New York, London, and Sydney, 1965, particularly pp. 576-649; and Pul and Paper Science and Technology, vol. I, Pulp, edited by C. Earl Libby, McGraw-Hill Book Co., New York, 1962, particularly chapters 9 and thereof; which references are hereby made a part of this application. As used herein, the term sulfite process includes bisulfite and sulfite processes utilizing ammonium, alkaliand akaline-earthmetal bisulfite or sulfite solutions under neutral or acidic conditions.

Although large quantities of chemical pulp are prepared by the sulfite process, this process possesses a numbee of inherent disadvantages. For example, the sulfite industry is very sensitive to the wood raw material that may be used. Additional disadvantages reside in the difficulty of recovering the cooking chemicals and utilizing the waste products which correspond to about one-half the wood substance. Obviously, if the chemicals are not recovered, means must be found for their disposal. The disposal of these waste liquors is a direct burden upon the sulfite economy, a factor often accentuated by laws prohibiting the disposal of these liquors in public waters. Although most wood species may be pulped by the sulfate process, the process is not entirely satisfactory. For example, the capital investment required per unit of production is very large.

It is, therefore, a principal object of the present invention to provide a sulfite process for the production of chemical pulp which obviates the disadvantages of the prior art processes.

It is another object of our invention to provide a pulp producing process whereby the quantity of the various chemicals used in the process can be reduced.

It is yet another object of this invention to provide a pulp producing process whereby the overall yield of chemical pulp is increased.

Other objects and advantages of the invention will become apparent as the description proceeds.

To the accomplishment of the foregoing and related ends, this invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

Broadly stated, the objects of this invention are accomplished by employing an N,N-dimethylamide of a straight chain carboxylic acid as a cooking aid during the digestion of the cellulosic material in the sulfate and sulfite processes.

Before proceeding with specific examples illustrating our invention, it may be well to indicate in general the nature of the amide required in the process.

A suitable N,N-dimethylamide of a straight chain carboxylic acid is prepared from a carboxylic acid contain ing 18 carbon atoms. Preferred acids are further characterized by having at least one carbon to carbon double bond, because such acids and the N,N-dimethylamides thereof are normally a liquid which is an aid in handling. It should be understood, however, that a saturated acid such as stearic may be used to prepare an eificient N,N- dimethylamide cooking aid. Generally, we prefer not to use the saturated acids because both the acid and the dimethylamide are solids at normal temperatures, causing handling difiiculties. Furthermore, the melting points f the N,N-dimethylamides of the saturated acids are relatively high and, as a consequence, these amides tend to form deposits in the papermaking system with a drop in temperature. Specific preferred acids include: oleic, linoleic, linolenic, ricinoleic, and mixtures thereof. Also suitable are the mixed acids found in tall, castor, corn, cottonseed, linseed, olive, peanut, rapeseed, safflower, sesame, and soybean oils which, of course, produce a mixture og N,N-dimethylamides. A mixture of carboxylic acids particularly suitable for use in our invention is that available commercially as tall oil fatty acids under the trademark Unitol ACD. A typical analysis of this product is as follows:

TABLE 1 Specification Typical range analysis Fatty acids, percent 98. 8-99. 7 98. 9 Rosin acids, percent 0.2-0. 6 0. 5 Unsaponifiables, perce 0 l 6 0.6 Linoleic acid, percent- 45 Oleic acid, percent 51 Saturated acid, percent 2. 0-2. 8 2. 4 Acid number 198-201 199 Saponification number 198-202 200 Color, Gardner 3- to 4- 3+ Viscosity:

S.S.U., F Gardner seconds 0. 9 Specific gravity, 60 F./60 F 0. 905 Titre, C 1 to 1.0 0. 0 Flash point, F- 375 Fire point, F 435 When the N,N-dimethylamide or a mixture thereof is used as a cooking aid in conjunction with the sulfite process for preparing chemical pulp, the dimethylamide may be conveniently and preferably added continuously into the high-pressure acid accumulator. Since most highpressure accumulators are equipped with a recirculating pump which continuously recirculates the sulfite solution, the N,N-dimethylamide may be added to the suction or to the discharge line of the pump. If the high-pressure accumulator is not provided with a recirculating pump,

the N,N-dimethylamide may be added to the low-pressure acid accumulator or to the raw acid storage tank. As another alternative, the N,N-dimethylamide may be siphoned into the cooking acid line going to the sulfite digester. When this procedure is followed, care must be taken to assure even distribution of the N,N-dimethylamide throughout the cooking acid. The concentration of N,N-dimethylamides actually used in the cooking process is not necessarily equal to the amount of the N,N-dimethylamide added to the system over the amount of cellulose fiber cooked during a given time. Since it is a practice to effect a side relief of the cooking acid back to the high pressure accumulator of 20-65 percent of the volume of acids pumped to the digester, then the concentration of the N,N-dimethylamide in the acid after equilibrium has been reached may be several times that obtained in a single cook evaluation. In the sulfate process, the N,N-dimethylamide or a mixture of such amides may be added to the white liquor going to the digester.

As to the amount of the N,N-dimethylamide to be added to the aqueous system, suitable and preferred quantities vary from 50 to 2,000 parts and 400 to 1,200 parts per million parts of moisture-free cellulose fiber, respectively. It will be understood, of course, that larger quantities of the N,N-dimethylamide may be used with no detrimental effect, but such larger quantities increase the costs of operation with limited material benefit.

We have found that the N,N-dimethylamides may be added to all types of sulfite systems, such as the conventional calcium (lime), milk of lime, sodium, magnesium and ammonia base acid sulfite pulping with highly beneficial results. In brief, the desirable results obtainable by following the teachings of our invention may be summarized as follows:

(1) Increased yield of usable or salable fiber per unit of wood. Specifically, a normal yield of unbleached pulp (so-called high-yield pulps as produced in newsprint mills) is about 45 percent based on the moisture-free weight of the Wood. Under the same conditions, the presence of the N,N-dimethylamide as a cooking aid increased the yield of unbleached pulp to more than 49 percent.

(2) Reduced consumption of cooking chemicals, such as sulfur, sodium, calcium, magnesium, and ammonia. This in itself serves to alleviate the problem of waste liquor disposal.

(3) Increased production per unit of equipment.

(4) Reduction in the quantity of rejects.

(5) Reduction in pitch content.

(6) Increase in overall mill efficiency in that a larger quantity of fiber of a higher quality is produced at a lower production cost.

(7) Steam savings. Both cooking time and temperature are reduced.

(8) Increase in brightness, particularly in high yield mills.

(9) Improved paper machine performance in that the dimethylamides have a desirable effect on papermill slug problems, because such slugs are usually high in sulfite fiber content.

In order to disclose the nature of the present invention still more clearly, the following illustrative examples will be given. It is to be understood that the invention is not to be limited to the specific conditions or details set forth in these examples except insofar as such limitations are specified in the appended claims.

EXAMPLE 1 In this example, two runs were made:

Run No. 1 was a conventional bisulfite digestion of coniferous wood chips for the production of chemical pulp to :be used for long fibers in the manufacture of newsprint as a control, and Run No. 2 was a duplicate of No. 1 with the exception that a mixture of N,N-dimethylamide was used as a cooking aid. The results of Run No. 1 are averages of data compiled during the six weeks prior to the trial Run No. 2. The results of Run No. 2 are averages of results obtained during the six days of trial Run No. 2.

In Run No. 1, 43 tons of air-dry coniferous wood chips were added to the digester and digested using sodium bisulfite prepared by bubbling S0 through an aqueous soda ash solution as the cooking acid. Approximate specifications of the cooking acid were 3.6 percent total and combined S0 at pH 4. The digesting procedure was as follows: Temperature raised to C. over a period of 30 minutes, held at that temperature for 1 hour, then raised to 145 C., and maintained at the latter figure for 5 hours. At the end of the 5-hour digesting period, the digester was blown and the contents discharged, product washed with water, cleaned, and dewatered.

In Run No. 2, the procedure of Run No. 1 was repeated with the exception that a mixture of N,N-dimethylamides of tall oil fatty acids was added to the digester at the beginning of the cooking in an amount equal to 800 parts per million parts of moisture-free cellulose fiber. The analysis of the tall oil fatty acids used to prepare the mixture of N,N-dimethylamides used is given in Table 1. A comparison of the control and the N,N-dimethylamides bisulfite cooks is summarized in Table 2.

The effectiveness of the mixture of N,N-dimethylamides prepared from Unitol ACD tall oil fatty acids as a cooking aid in reducing cooking time was determined by comparing the results obtained by carrying out a series of cooks in the presence of the dimethylamide to similar cooks in the absence of the dimethylamide.

The particular mill at which this test was conducted has four digesters and produces approximately 10 tons (1 ton=907 kilograms) of air-dried (10% H 0) cooked sulfite cellulose pulp per cooking cycle or blow. Normal production (no dimethylamide used) has been 12 to 13 cooks (blows) per day, using a cooking cycle as follows:

1.0 hour from capping digester to C.

1.25 hours from 110 C. to 163 C.

3.25 hours at 163 C. (normal cooking temperature). Total cooking time: 5 .5 hours.

The cooking acid used was a magnesium base sulfite liquor containing 5.2 percent total S0 with a pH of about 3.8.

In this series of cooks, a mixture of the dimethylamides as identified above was added to the cooks in an amount equal to 800 parts per million parts of air-dried pulp. It was found that the use of the dimethylamide cooking aid made it possible to reduce the cooking time from 5.5 to 4.75 hours. This reduction in cooking time was obtained with no sacrifice in yield or quality of the final sulfite cellulose pulp. Overall production was increased to an average of 14 to 15 cooks per day.

The results obtained in Example 2 as compared to those in Example 1 were as follows:

(1) Reduced cooking time.

(2) Reduced acid consumption.

(3) Reduced rejects.

(4) Reduced pitch content of the pulp.

(5) Pitch did not deposit on screen plates.

(6) Easier screening.

(7) Increased yield (more tons per blow).

(8) Increased production (more tons per blow, more blows per day).

(9) Improved pulp strength as measured by bursting test and tearing test.

EXAMPLE 3 The procedures of Examples 1 and 2 were repeated in which the N,N-dimethylamides of the mixed acids found in castor, corn, cottonseed, linseed, olive, peanut, and soybean oils were used instead of the N,N-dimethylamides of tall oil fatty acids. The results were similar to those obtained in Examples 1 and 2.

We have also found that the advantageous results of our invention are attained when the N,N-dimethylamides of the carboxylic acids listed-are used as a cooking aid, in aqueous ammonium, alkaliand alkaline-earth metal bisulfite and sulfite solutions under either mildly alkaline, neutral, or acidic conditions.

EXAMPLE 4 In this example, two runs were made:

Run No. 1 involved the digestion of hardwood chips for the production of chemical pulp to be used for fibers in the manufacture of corrugating medium by a conventional continuous process operating at a production rate of 220 tons of pulp per day. Digestion conditions were as follows: pressure 150 p.s.i.g., temperature 366 F., and a reaction time of 16 minutes. The cooking liquor which contained 1 pound of sodium sulfite and 0.45 pound of sodium carbonate per gallon was added to the digester at a rate of 37 gallons per minute. Consistency in the primary refiners was about 28 percent and in the secondary refiners it ranged from 4.6 to 5.1. Freeness averaged about 465 (Canadian Standard) out of the secondary refiners.

In Run No. 2, the procedure of Run No. 1 was repeated with the exception that a mixture of tall oil fatty acids (same as used in Run No. 2 of Example 1) was introduced into the cooking liquor on the intake side of the pump delivering the liquor to the digester at a rate of about 300 parts per million parts of moisture-free cellulosic fiber.

It was found that the addition of the dimethylamides to the digester reduced power consumption by about percent and increased the production of chemical pulp by about 15 percent with no sacrifice in yield or quality of the final chemical pulp.

EXAMPLE 5 The effectiveness of the mixture of N,N-dimethylamides prepared from Unitol ACD tall oil fatty acids as a cooking aid in reducing cooking time, thus increasing production in continuous digestion process (kraft process), was determined by comparing the results obtained by carrying out the cooking process in the absence of the dimethylamides (Run No. 1) to a similar cooking process in the presence of the dimethylamides (Run No. 2).

In Run No. 1, about 300 tons per day of air-dry chips consisting of about 60 percent mixed pines including ponderosa, sugar, western white, and lodge pole pine; percent Douglas-fir; and 15 percent white fir was added to the continuous digester and digested using a cooking liquor containing active alkali, 1.5 pounds per cubic foot; eifective alkali, 1.3 pounds per cubic foot; and a sulfidity of 20 percent. Foregoing numerical values are as measured in the cooking zone. Total alkali in the white liquor used for makeup was 7.2 pounds per cubic foot. Cooking conditions were as follows: Temperature in the second zone of the digester, 280 F., increased to 350 F.; and in the third zone (where cooking actually takes place), 350 F. Contact time in the third zone was 45 minutes. The liquor consistency or ratio of the cooking liquor to wood at the discharge of the digester was 18.5 percent. There was a circulation rate of 325 gallons per minute of cooking liquor in the third zone, of which gallons per minute was made up from fresh white liquor.

In Run No. 2, the procedure of Run No. 1 was repeated wtih the exception that a mixture of tall oil fatty acids (same as used in Run No. 2 of Example 1) was introduced into the cooking liquor on the intake side of the pump delivering the liquor to the digester at a rate of about 800 parts per million parts of moisture-free cellulosic fiber.

It was found that the addition of the dimethylamides to the digester increased the rate at which the chemical pulp was produced by about 12 percent with no sacrifice in yield or quality of the final chemical pulp.

In addition, the use of the dimethylamides as a cooking aid inhibited scale formation both in the digesters and in the heaters located outside the digesters. This result was apparent from the fact that in Run No. 1 it was necessary to remove the scale formed in the heaters after 1.5 days operation, while in Run No. 2 where the dimethylamides were used, the heaters could be operated for a period of 7 days before the necessity of scale removal.

Beneficial results are also attained when the N,N-dimethylamides as disclosed herein are added to other processes such as the soda process of producing chemical pulp and the production of groundwood from bolts or chips after chemical impregnation followed by defibering with conventional stone grinders or disc refiners or various modifications thereof.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

1. In a process of preparing a chemical pulp from Wood chips by digesting said cellulosic material in an aqueous solution containing a pulping chemical under alkaline, neutral, or acidic conditions at an elevated temperature, the improvement which comprises incorporating in said aqueous solution a composition comprising an N,N-dimethylamide of a straight chain carboxylic acid containing 18 carbon atoms in an amount varying from about 50 to 2,000 parts per million parts of moisturefree wood chips.

2. The process of claim 1 wherein the chemical pulp is prepared from the wood chips by the sulfite process.

3. The process of claim 1 wherein the chemical pulp is prepared from the wood chips by the kraft process.

4. The process of claim 1 wherein the pulping chemical is ammonium, alkalior alkaline-earth-metal bisulfite or sulfite.

5. The process of claim 1 wherein the straight chain carboxylic acid contains at least one carbon to carbon double bond.

6. The process of claim 5 wherein the straight chain carboxylic acid is a mixture of straight chain carboxylic acids containing 18 carbon atoms and at least one carbon to carbon double bond.

7. The process of claim 5 wherein the straight chain carboxylic acid is a mixture of acids derived from tall oil.

8. The process of claim 5 wherein the straight chain carboxylic acid is a mixture of acids derived from corn oil.

9. The process of claim 5 wherein the straight chain carboxylic acid is a mixture of acids derived from cottonseed oil.

10. The process of claim 5 wherein the straight chain carboxylic acid is a mixture of acids derived from linseed oil.

11. The process of claim 5 wherein the straight chain carboxylic acid is a mixture of acids derived from peanut oil.

12. The process of claim 5 wherein the straight chain carboxylic acid is a mixture of acids derived from soybean oil.

References Cited UNITED STATES PATENTS 9/1953 Van Dyk 162-75 11/ 1955 Lourd 162-75 4/ 1956 Corbin 162-72 HOWARD R. CAINE, Primary Examiner.

US. Cl. X,R. 

